Downsides to salt pools

[waste]

Hi all! Nater and Richard have requested my 'take' on this, here it comes:

Davenj (post #2) is wondering about the 'white sludge' in his anchor cups - DavidD and I talked about it in http://www.poolforum.com/pf2/showthread.php?t=6237 (#5+)

I have also been in 2 threads discussing SS rusting (http://www.poolforum.com/pf2/showthread.php?t=5114 / http://www.poolforum.com/pf2/showthread.php?t=3310)

The 'bolded' section in the last post may well have something to do with PatL34's admonition against using dry acid (sodium bisulfate) in SWCG pools.

Aside from this, I don't really have much to say, if there's been any degridation of deck or equipment - I haven't noticed it, but we've only been using these units for a few years (the co. I worked for in Va used the Lectronator and did mostly shotcrete pools w/ precast coping- but that was 12 years ago and I wasn't looking for premature failing due to salt and my memory isn't so great that I can remember if any of those pools had coping or deck problems at the ladders or stairs)

The only other thing I've noticed with salt pools is the accumulation of salt 'crust' at the exit areas, and rust on NON STAINLESS bolts on deck chairs, etc where people sit with salinated dripping wet bathing suits.

One thing is for sure; next season I'm going to take a very close look at the deck areas which are routinely exposed to the water from a SWCG pool! (ok, so this is the 'long term approach', but good studies are done over long periods - BTW, who's funding this study )


{I've been 'subscribed' to this since it went to page 2 and will follow it through - it's an excellent discussion on a possible pitfall to having a salt pool} - Waste

[chem geek]

Thanks waste. What I think is important is to look out for corrosion in both salt and non-salt pools and to take careful measurements of water chemistry parameters when such corrosion is found. We don't want to bias ourselves by only "looking" for corrosion in salt pools. I knew about your response to one of the two corrosion links I had, but didn't know about the other. Thanks for those references -- every piece of information helps to put the puzzle together, including the reference to avoiding sulfates (dry acid) in some situations.

I reread this statement from the EPA document that I quoted earlier, and this time am including a different section in bold.

Non-halide salts have little effect on stainless steels, but chlorides particularly tend to promote pitting, crevice corrosion, and stress-corrosion cracking. In some cases sulfates seem to aggravate the effects of chlorides. Chlorides present in amounts as little of 0.3% with sulfates present can produce severe corrosion. Even quite low concentrations of chlorides can cause corrosion when concentrated by occlusion in surface films. Oxidizing chlorides such as ferric or cupric chloride are specific for severe pitting, although halide salts can cause severe pitting and stress corrosion cracking. The austenitic stainless steels are, however, the most susceptible of all the stainless steels to “chloride” stress corrosion cracking.

The term "oxidizing chlorides" may not be a specific issue with having only compounds of chloride. It may also occur with the combination of a strong oxidizer in the presence of chloride, but this is just my speculation. Interestingly, the "ferric" form of iron is indeed an oxidizer with a rather high (standard) reduction potential of +0.771V while the "cupric" form of copper is a much weaker oxidizer with a low reduction potential of +0.3419V. By comparison, oxygen has a very high reduction potential of +1.229V while hypochlorous acid has an even higher reduction potential of +1.482V. One can only compare these potentials from a molar equivalent basis and in actual pool water the actual potentials (based on actual concentrations or activities) are completely dominated by hypochlorous acid (hypochlorite ion is tied to this as well, with a lower +0.81V molar reduction potential) followed by dissolved oxygen. In other words, there may be a very strong effect between chlorides and both sulfates and oxidizers (e.g. chlorine) in terms of severe corrosion (pitting) of stainless steel. The reference to "halide salts" would include sodium chloride (and other metals with chloride, including iron and copper already mentioned), but this paragraph from the EPA is wholly qualitative and not quantitative and therefore unsatisfactory. It can guide us for what questions to ask, but does not give us specific answers.

Richard

[waste]

Richard, (as often happens when I try to keep my 'word count' down), I misrepresented what I was trying to say (go figure ). Though I specificly said I'd look at the SWCG pool areas, it was my own 'shorthand' for 'looking' at them compared to pools of the same age and ~ usage which diddn't have the SWCGs. This also raises the issue of my not being 'blind' (ie 'double-blind experiment') - undoubtably, I'll report some more or less corrosion to the pools in question. However, since I work in a relatively small area, many other factors will be removed from my observations. (in my 'miss-spent' youth, I studied psychology, and as it's a 'soft science', they made sure to 'pound-in' the necessity of using the 'hard science' methodology)

[Waterworks]

I just found another potential problem, or at least a very good source for testing. A residential customer of ours with an indoor salt pool called and said his ladders and light were corroding, so I went to check it out with my service man. The ladders were so corroded that they were falling apart. His pool is full-tile and his original white grout lines are now black, his original blue tiles are now greenish. The light ring is black also. We went and checked his cartridge filters and they were black, caked with rust. The pump basket was also black. The ladder rails that are above water are rusting, as you'll see in the pictures, but nowhere near as bad as the underwater parts.
Outside the pool there is a slide, metal deck drains, metal exhaust vents, tile decking, tile safety grip coping and a flagstone deck section with grout. The slide posts showed no corrosion, but I think they are aluminum. One of the metal deck drains is showing some rust, one is just fine. the metal exhaust vents show no corrosion. The flagstone and tile deck sections show no corrosion.

We installed the SWG in May 2005. The pool was installed around 2000. Before the SWG came along there was no corrosion. We went on a service call in March of last year to clear up the pool, as it was cloudy. At that point, the generator had been turned all the way down. We had not had any complaint of corrosion, and did not actively look for it at this point. My service man turned it up to 30% and told the customer to keep checking the chlorine levels and to keep them high until the pool was clear again, then turn the SWG down. The customer has not brought in a water sample since, and while I was speaking with him today he did not know where his test kit was. When I looked at the SWG today, it was still set on 30%. The pool room had absolutely no chlorine smell and was not hard on my eyes. His kids use the pool almost every day and have never complained about anything.

Here are the results of my water test:
Chlorine - I used my FAS-DPD test kit. I filled the sample to 10ml, then added two scoops of powder. As the powder hit the water it turned bright pink, but then went back to clear. As I shook the sample, it would periodically turn to pink, but then back to clear. I'd seen this before so I added my R0871 and after 25 drops the sample had turned pink and stayed that way. I kept adding drops until I had done 110. Then I mixed 1 part pool water with 3 parts distilled water and it still took 100 drops in a 10 mL test, meaning a chlorine reading of somewhere between 55-100 ppm. I did an OTO test and the sample turned bright red.
pH - phenol red test was off the charts so I used my Hannah meter and got 8.4
TDS (Our city water is about 650ppm) - 2850
Salt - 3200 on the Aquarite machine and 3000 on my Taylor Kit
Alkalinity - 80
Calcium - 140
CYA - 0

It seems to me that this info coincides with what I already thought. Most SWG's create WAY too much chlorine for indoor pools. This excess chlorine causes corrosion. Since this corrosion ties in with the installation of the SWG, the SWG takes the blame.

I brought one of his ladders back to my shop so I could try to do some kind of test on it. I was thinking of setting up 2 buckets, one with high chlorine and one with high salt and setting each leg of the ladder (because they are evenly corroded at this point) in it's own bucket. I would take pictures before and after and compare the effects. I would also test the water every day or two and keep it balanced. I wish I had a way to test a third part so I could do high chlorine with CYA. Does anyone think this test would be worthwhile, and does anyone have any tips to make the test better?

If anyone can tell me how to post a picture, I will post a couple pics of the ladder.

Brad

[chem geek]

I brought one of his ladders back to my shop so I could try to do some kind of test on it. I was thinking of setting up 2 buckets, one with high chlorine and one with high salt and setting each leg of the ladder (because they are evenly corroded at this point) in it's own bucket. I would take pictures before and after and compare the effects. I would also test the water every day or two and keep it balanced. I wish I had a way to test a third part so I could do high chlorine with CYA. Does anyone think this test would be worthwhile, and does anyone have any tips to make the test better?

If anyone can tell me how to post a picture, I will post a couple pics of the ladder.

Brad, I think that this forum is now limiting uploading of pictures, but if you send them to me (via E-mail -- click on my name on any post and there will be a link to E-mail to me) then I can host them on the website I use for the other pool stuff I have and then your post can link to that.

Sorry to hear about this customer, but man, that much chlorine with no CYA is horribly corrosive. As for what test to do vis-a-vis salt, part of what you want to compare is moderate chlorine levels (say, 5 ppm FC with no CYA) with salt at 3000 ppm vs. the same chlorine levels with salt at around 300 ppm. We already know and understand the effects of CYA on chlorine so that isn't as useful. What we really want to know is whether the salt in the presence of chlorine accelerates the corrosion so you want the same chlorine level in both buckets with the only difference between the two being the salt level. To make this a reasonably fair test, go ahead and add baking soda to raise the TA and maybe even some calcium chloride for CH. That would make it very much like pool water (so try and keep the pH around 7.5 as well). Whenever you test or add chemicals, be sure and do the same sort of mixing (circulation) in the two buckets as that also affects corrosion (if there is no circulation, then the buildup of "rust" products probably slows down additional corrosion).

This story really emphasizes the need for customer education about SWG and especially about testing your water regularly. I can hear Carl's words now...

Stay ahead of your water!
Take 2 to 5 minutes every day for pool maintenance!

Richard

[chem geek]

I thought of another possible reason why SWG manufacturers recommend high levels of CYA (typically 70-80 with 80 "best"). In this thread (starting at around post #30) I said that the reason for high CYA in SWG pools is that this is more efficient at producing chlorine. Though this appears to be true (based on pool owner's experiences with runtime), I didn't say what happens when there is a buildup of chlorine at the plate. In addition to production of chlorine, there is also a competing reaction of producing oxygen. If the reaction producing chlorine slows down, then the reaction rate of producing oxygen can speed up in its place, though the production of oxygen is normally less favored than that of chlorine (technically, the production of oxygen is more likely to occur in an equilibrium sense, but it has a much higher activation energy than the production of chlorine so results in an overvoltage such that chlorine is more favored than oxygen).

It is interesting to note that different SWG systems use a different voltage to drive their cells. Some operate in the range of 6-9 volts, but others are in the range of 22-30 volts. Though the voltage level required to only produce chlorine and not oxygen is quite low (perhaps between 1.16V and 1.45V at normal pool concentrations of pH and chlorine), higher voltages will probably produce both chlorine and oxygen with chlorine being produced more. I do not know if higher voltages change this relationship, but suspect that it might, with the oxygen proportion increasing somewhat at higher voltages (though still produced in lower quantity than chlorine). At any rate, having lower CYA levels may very well increase the oxygen proportion and this can be dangerous since the other plate is producing hydrogen. Though hydrogen by itself is not explosive and in water does essentially nothing, the combination of oxygen and hydrogen can be dangerous if this combination of gasses were to accumulate (if the pump shuts off but the SWG cell keeps running, for example).

The blog link at the first post in this thread has been reporting some explosions of SWG systems and I wonder if those pools were using lower CYA levels than recommended and produced more oxygen that could have been at explosive levels with hydrogen. It is also possible that the explosions weren't actual ignitions of hydrogen and oxygen, but high pressure bursts from the buildup of gas pressure. It should be understood that under normal operating conditions when the SWG only operates when the pump is running, that the risk of explosion is virtually nil. I'm just trying to sort out some unusual circumstances (where the salt cell didn't shut off when the water flow stopped) to better understand what is going on.

[EDIT] A further note: Even the most dangerous levels of hydrogen gas and oxygen gas in a 2:1 ratio will not spontaneously explode unless the pressures are very high (much higher than what pipes would handle, so bursting would occur before explosion) or the temperatures are very high (at least 400C) so generally some sort of spark or flame is required to trigger a literal thermal explosion. In other words, the risk of oxygen production and explosion is low; the risk of a pressure burst is high if the salt cell operates in a closed valve environment (which would only occur if some component failed to operate properly). [END-EDIT]

Richard

[giroup01]

At any rate, having lower CYA levels may very well increase the oxygen proportion and this can be dangerous since the other plate is producing hydrogen.

Richard,

Can you expand a bit on the chemistry behind low CYA levels favoring O2 production ?

Thanks,

[Gunslinger]

Richard, your explanations of SWG chemistry are truly awesome, but I don't believe for NY second that the reason for the manufacturers' 70-80ppm CYA recommendation (I've actually seen 80-100ppm recommended) has anything to do with real-world chemistry. Rather, I suspect it has more to do with a slightly different interpretation of the acronym "CYA", rooted in the early days of the SWG evolution when reliable operation was not up to par with current devices. In a loss-of-power event, for example, high CYA levels would help to ensure that all your residual FC doesn't disappear on a sunny day before you've noticed something is amiss. I believe there is no longer any rationale justification for this recommendation other than my suspicion that the manufacturers have no interest in pursuing the matter and are content to leave it unchanged.
Personally, I barely manage to keep my CYA level above 25ppm, and have experienced no consequences from that during the four seasons I have operated my Pool Pilot SWG -- although I do test my FC and Ph quite often.